Biosensors are analytical tools that can be used to measure the presence of a single molecular species in a complex mixture by combining the exquisite molecular recognition properties of biological macromolecules with signal transduction mechanisms that couple ligand binding to readily detectable physical changes (Hall, Biosensors, Prentice-Hall, Englewood Cliffs, New Jersey; Scheller et al., Curr. Op. Biotech. 12:35-40, 2001). Ideally, a biosensor is reagentless and, in contrast to enzyme-based assays or competitive immunoassays, does not change composition as a consequence of making the measurement (Hellinga & Marvin, Trends Biotech. 16:183-189, 1998). Most biosensors combine a naturally occurring macromolecule such as an enzyme or an antibody, with the identification of a suitable physical signal particular to the molecule in question, and the construction of a detector specific to that system (Meadows, Adv. Drug Delia-. Rev. 21:177-189, 1996). Recently, molecular engineering techniques have been explored to develop macromolecules that combine a wide range of binding specificities and affinities with a common signal transduction mechanism, to construct a generic detection system for many different analytes (Hellinga & Marvin, Trends Biotech. 16:183-189, 1998).
Escherichia coli periplasmic binding proteins are members of a protein superfamily (bacterial periplasmic binding proteins, bPBPs) (Tam & Saier, Microbiol. Rev. 57:320-346, 1993) that has been shown to be well suited for the engineering of biosensors (U.S. Pat. No. 6,277,627). These proteins comprise two domains linked by a hinge region (Quiocho & Ledvina, Molec. Microbiol. 20:17-25, 1996). The ligand-binding site is located at the interface between the two domains. The proteins typically adopt two conformations: a ligand-free open form, and a ligand-bound closed form, which interconvert via a hinge-bending mechanism upon ligand binding. This global, ligand-mediated conformational change has been exploited to couple ligand binding to changes in fluorescence intensity by positioning single, environmentally sensitive fluorophores in locations that undergo local conformational changes in concert with the global change (Brune et al., Biochemistry 33:8262-8271, 1994; Gilardi et al., Prot. Eng. 10:479-486, 1997; Gilardi et al., Anal, Chem, 66:3840-3847, 1994; Marvin et al., Proc. Natl. Acad. Sci. USA 94:4366-4371, 1997, Marvin and Hellinga, J. Am. Chem. Soc. 120:7-11, 1998; Tolosa et al., Anal. Biochem. 267:114-120, 1999; Dattelbaum & Lakowicz, Anal. Biochem. 291:89-95, 2001; Marvin & Hellinga, Proc. Natl. Acad. Sci. USA 98:4955-4960, 2001; Salins et al., Anal. Biochem, 294:19-26, 2001). Conformational coupling mechanisms can also be devised to alter the flow of current between the surface of an electrode derivatized with the engineered bPBP containing a covalently attached redox cofactor (Benson et al., Science 293:1641-1644, 2001).
The present invention provides a method of utilizing bPBPs to generate biosensors for a variety of chemical classes including sugars, amino acids, dipeptides, cations, and anions. These biosensors have widespread utility including in clinical, industrial, and environmental settings.